Calculator demo

The following is the ruby version of calculator:

As simple as that!

S-expression demo

The above example utilizes the pre-built Expressions class to help building expression parser. Another example is for a simpler s-expression parser (in lisp syntax, "- (+ 1 (* 2 2)) (1)" sort of thing). As s-expression is way simpler to parse than expressions with infix operators, we will build the parser without using Expressions class:

The code is pretty self-explanatory. The only thing to note is that we had to manually skip whitespaces using the "ignore" function. (Whitespace skipping can be more elegant if we do a 2-phase parsing, where lexer takes care of them before grammar rule kicks in. Such use is demonstrated in detail in the test code.)

More examples

Take a look at the SQL Parser for a more sophisticated parser, where more traditional lex/parse approach is used.